Using Pop2OpenCRM, you can link emails to Projects, Opportunities and Campaigns.
This is done by using the Email Address field on those modules.
If you populate this with an email address, any emails sent to that address will automatically be linked to the relevant entity.
For example, you may wish to set up an email address for feedback to a specific Project: firstname.lastname@example.org and then track all of the emails sent to that address in OpenCRM. This feature allows you to do that.
The pre-requisite is that you need to be using Pop2OpenCRM to convert your inbound emails. For more information on Pop2OpenCRM please refer to this FAQ.